This invention relates in general to high data rate (broadband) radio frequency communications, and more particularly to frequency re-use cellular plans that minimize the interference sensitivity of such communication.
In the past, high speed information communication between processor-based systems, such as local area networks (LAN) and other general purpose computers, separated by significant physical distances has been an obstacle to integration of such systems. The choices available to bridge the physical gap between such systems have not only been limited, but have required undesirable tradeoffs in cost, performance, reliability, and service deployment time.
One group of historically available communication choices includes such solutions as the utilization of a standard public switch telephone network (PSTN) or multiplexing signals over an existing physical link to bridge the gap and provide information communication between the systems. Although such solutions are typically inexpensive to implement, they include numerous undesirable traits. Specifically, since these existing links are typically not designed for high speed data communication, they lack the bandwidth through which to communicate large amounts of data rapidly. As in-building LAN speeds increase to 100 Mbps, the local PSTN voice grade circuits even more markedly represent a choke point for broadband metropolitan area access and therefore are becoming a less and less desirable alternative. Furthermore, such connections lack the fault tolerance or reliability found in systems designed for reliable transmission of important processor-based system information.
Another historically available group of communication choices is found at the opposite end of the price spectrum than those mentioned above. This group includes such solutions as the utilization of a fiber optic ring or point-to-point microwave communication. These solutions are typically cost prohibitive for all but the larger users. The point-to-point systems require a dedicated system at each end of the communication link which lacks the ability to spread the cost of such systems over a plurality of users. Even if these systems were modifiable to be point-to-multipoint, to realize the economy of multiple system use of some system elements, the present point-to-point microwave systems would not provide broadband data services but rather traditional bearer services such as T1 and DS3. Furthermore these systems typically provide a proprietary interface and therefore do not lend themselves to simple interfacing with a variety of general purpose processor-based systems.
Although a fiber optic ring provides economy if utilized by a plurality of systems, it must be physically coupled to such systems. As the cost of purchasing, placing, and maintaining such a ring is great, even the economy of multi-system utilization generally does not overcome the prohibitive cost of implementation.
Accordingly, point-to-multipoint systems such as shown and described in above referenced U.S. Pat. No. 6,016,313 have been developed to provide broadband communication infrastructure in an efficient and economical alternative. For example, a preferred embodiment point-to-multipoint system described in the U.S. Pat. No. 6,016,313 provides for a network of point to multipoint hubs to establish cellular type coverage of a metropolitan area. Such systems are generally more economical to deploy than systems such as fiber optic networks, due to their use of wireless links avoiding the cost of laying fiber to all nodes on the network, and point-to-point microwave, due to the sharing of resources among several or many users.
The problem addressed by the invention is the limited spectrum available for use in the aforementioned point-to-multipoint, or similar, systems. The invention provides for dense re-use of frequencies in the spectrum while still providing desirable signal quality, e.g. high signal/noise ratio and/or low interference. Moreover, the preferred embodiment of the invention provides for efficient use of the spectrum such that portions of the spectrum remain available for use in special applications, such as providing unique channels for implementing link redundancy and/or providing unique channels for use in point interference situations.
These and other objects, features and technical advantages are achieved by a system and method which provides a cellular reuse plan. The invention allows for a tessellating (repeating in the form of a mosaic pattern) grouping of cells that have controlled carrier to interference limits that meet acceptable carrier to interference (C/I) levels. The invention preferably uses geometry theory in the grouping of the cells, (see Introduction to Geometry, by H. S. M. Coxeter, Wiley, 2nd edition 1969, which is incorporated herein by reference).
The invention preferably uses a uniformly rotated set of identical or substantially identical cell assignments within a mosaic repeat pattern. The rotated repeating cell is used because of practical limits on the number of available frequency, or other channel, assignments. The number of assignments is preferably associated with the allocated bandwidth and required data throughput of subscribers.
The preferred embodiment of the invention operates in line of sight (LOS) system, and preferably uses polarization discrimination between ones of the adjacent cell sectors to thereby provide additional orthogonal spectrum (note that polarization works for non-line of sight also). The mosaic pattern of the preferred embodiment uses several different cell types. Each cell type of the preferred embodiment comprises a different set of frequency, or other channel, assignments. The different cell types may be arranged or deployed in the mosaic pattern.
The cells of the present invention may comprise hexagonal or square shapes, the mathematics for which are discussed in Advanced Mobile Phone Service, Bell Systems Technical Journal, January 1979, and Microwave Mobile Communications, by W. C. Jakes, IEEE Press, reissued 1993, both of which are hereby incorporated herein by reference. Note that other cell shapes could be used according to the present invention, for example circular, triangular, octagonal, etcetera.
The preferred embodiment of the invention allows for the deployment of both FDD (frequency division duplex) and TDD (time division duplex) PTM (point to multipoint) cellular systems under conditions of controlled, and acceptable, levels of intra-system interference. For TDD systems, preferred embodiments of the present invention have no burst synchronization requirement between cells, as well as no transmit/receive symmetric transmission requirement between different sectors in a cell or different sectors within a multi-cell system, i.e., outbound/inbound sector transmission duration can be dynamically adapted in each sector to meet user requirements.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.